In recent years, the introduction of optical subscriber (access) systems primarily as communication systems with a 1 Gbps-class transmission capacity for a passive double-star cable plant capable of accommodating multiple subscribers, such as Gigabit Ethernet (registered trademark)-Passive Optical Network (GE-PON) and G-PON, has progressed.
As a next-generation communication system, communication systems with a transmission capacity of 10 Gbps have been under consideration, and active efforts have been made toward standardization of the same. In addition, discussions have been held concerning migration to a 10-Gbps communication system newly mixed with an existing 1-Gbps communication system (refer to Japanese Laid-Open Patent Application Publication No. H8-8954; and Tanaka, Keiji, “Backward Compatibility”, 10 Gb/s PHY for EPON Study Group IEEE 802.3 Interim Meeting Austin, Tex., Presentation Materials, [online], May 24 to 25, 2006, KDDI R&D Laboratories Inc. [searched on Nov. 29, 2006], Internet <URL:http://grouper.ieee.org/groups/802/3/10GEPON_study/pub lic/may06/tanaka—1—0506.pdf, for example).
FIG. 9 is a diagram for explaining a configuration example of a communication system without a mixed 1-Gbps and 10-Gbps communication systems environment. As depicted in FIG. 9, in a first configuration example, the 1-Gbps communication system and the 10-Gbps communication system are configured independently. A 1-Gbps optical line terminal (OLT) is connected only to 1-Gbps subscriber optical network units (ONUs). A 10-Gbps optical line terminal is connected only to 10-Gbps optical network units.
FIG. 10 is a diagram for explaining a first configuration example of a communication system having a mixed 1-Gbps and 10-Gbps communication systems environment. As depicted in FIG. 10, in the first configuration example, the 1-Gbps communication system and the 10-Gbps communication system are mixed through wavelength division multiplexing (WDM).
FIG. 11 is a diagram for explaining a second configuration example of a communication system having a mixed 1-Gbps and 10-Gbps communication systems environment. As depicted in FIG. 11, in the second communication example, the 1-Gbps communication system and the 10-Gbps communication system are mixed through time division multiple access (TDMA).
FIG. 12 is a diagram for explaining a third configuration example (downlinks) of a communication system having a mixed 1-Gbps and 10-Gbps communication systems environment. As depicted in FIG. 12, in the third configuration example, the 1-Gbps communication system and the 10-Gbps communication system are mixed through WDM for downlinks from OLTs 1242 and 1245 to optical network units 1211 and 1212.
Optical signals from the 1-Gbps optical line terminal 1242 and 10-Gbps optical line terminal 1245 are transferred through WDM via a branching unit 1241 and a transmission line 1230, and are transmitted by a branching unit 1220 to the 1-Gbps optical network unit 1211 and the 10-Gbps optical network unit 1212. Among the optical signals output from the branching unit 1220, the optical network unit 1211 and the optical network unit 1212 receive only signals of optical wavelengths allocated thereto, respectively, according to system speed.
FIG. 13 is a diagram for explaining the third configuration example (uplinks) of a communication system having a mixed 1-Gbps and 10-Gbps communication systems environment. As depicted in FIG. 13, in the third configuration example, the 1-Gbps communication system and the 10-Gbps communication system are mixed through TDMA for uplinks from the OLTs 1211 and 1212 to the optical network units 1242 and 1245.
The optical network units 1211 and 1212 transmit optical signals within respective time frames allocated thereto. Optical signals from the optical network unit 1211 and the optical network unit 1212 are transferred through TDMA via the branching unit 1220 and the transmission line 1230, and are transmitted by the branching unit 1241 to the OLTs 1242 and 1245. Among the optical signals output from the branching unit 1241, the optical line terminal 1242 and 1245 receive only signals within the time frames allocated thereto, respectively, according to system speed.
However, the conventional technique depicted in FIGS. 12 and 13 cause optical coupler branch loss at the branching unit 1241 (>3 dB) in both the 1-Gbps communication system and the 10-Gbps communication system, which leads to a problem in that the existing loss budget cannot be maintained for the 1-Gbps communication system or the 10-Gbps communication system.
Particularly in the 1-Gbps communication system, apparatuses that are currently being introduced have no loss budget margin, and when migration to the 10-Gbps communication system takes place, the apparatuses cannot be operated due to optical coupler branching loss. Thus, a problem further arises in that the system requires fundamental restructuring such as increasing optical output from a cable plant, reducing the capacity for accommodation of subscribers in the 1-Gbps communication system, etc.